Attach safety device (4)
I believe the answer is:
fuse
This might be a double definition.
'attach' is the first definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'safety device' is the second definition.
(electronic safety device)
Can you help me to learn more?
(Other definitions for fuse that I've seen before include "Join or blend to form an entity" , "Coalesce, melt together" , "'Blend, merge (4)'" , "Melt with heat" , "Common electrical safety device in homes etc" .)